Active directory rename - root to sub domain

It would be just a rename still. ad.domain.com isn't a subdomain (really child domain in AD terms) technically in your instance. It would be the root domain name in your AD environment. You don't have to create domain.com first in order to use ad.domain.com.

And yes, definitely better to go with something like ad.domain.com than use the same internal and external DNS name.

As long as you don't have Exchange, this is fine.

ad.domain.com would be better than domain.com

http://technet.microsoft.com/en-us/library/cc738121%28WS.10%29.aspx

Windows Active Directory naming best practices?